效果如图
此图只实现 左下与右下边角样式
右上与左上同理

/*  容器 */
.card-mini {position: relative;
}
/*  左下*/
.card-mini::before {content: '';position: absolute;left: 0;bottom: 0;width: 20px;height: 20px;border-bottom: 2px solid #253d64;border-left: 2px solid #253d64;border-bottom-left-radius: 10px;
}
/*  右下*/
.card-mini::after {content: '';position: absolute;right: 0;bottom: 0;width: 20px;height: 20px;border-bottom: 2px solid #253d64;border-right: 2px solid #253d64;border-bottom-right-radius: 10px;
}

右上与左上 将上面的bottom改为top

.card-mini::before {content: '';position: absolute;left: 0;top: 0;width: 20px;height: 20px;border-top: 2px solid #253d64;border-left: 2px solid #253d64;border-top-left-radius: 10px;
}
.card-mini::after {content: '';position: absolute;right: 0;top: 0;width: 20px;height: 20px;border-top: 2px solid #253d64;border-right: 2px solid #253d64;border-top-right-radius: 10px;
}